A structured light is a light with a specified pattern. Examples of the structured light include a linearized beam, a beam with a stripe pattern, or a beam with a grid pattern when the light is projected on a screen at some distance away. With increasing development of optical technologies, the structured light can be designed to have a linear pattern with narrower linewidth, a planar pattern with specially defined uniformity, a grid pattern with specified wide angle, or even a more complex light pattern. Consequently, the structured light can be applied to many fields such as 3D contour regeneration, distance measurement, anti-counterfeiting recognition, and so on. In other words, the importance of the structured light is gradually recognized, and it is useful to closely evaluate the associated technologies of structured light generation.
FIG. 1 is a schematic side view illustrating an inner portion of a conventional structured light generation unit. As shown in FIG. 1, the structured light generation unit 1 comprises a light source 11, a collimating lens 12 and a diffractive optical element (DOE) 13. The light source 11 can emit plural light beams 111. The collimating lens 12 is arranged between the light source 11 and the diffractive optical element 13. The collimating lens 12 is used for collimating the plural light beams 111. A special pattern (projection pattern) 131 is embedded in the diffractive optical element (DOE) 13 to provide the structured light when the light beams 111 are incident thereon. The functions of DOE are achieved according to a diffraction theory, and the pattern can be quite complicated. Consequently, the coherent or partially coherent light can interact to form a new form of light. After the plural light beams 111 pass through the collimating lens 12 and the projection pattern 131, the structured light generation unit 1 generates a structured light 110 corresponding to the projection pattern 131.
The operations of the structured light generation unit 1 will be illustrated as follows. FIG. 2 is a schematic side view illustrating a structured light outputted from the conventional structured light generation unit. According to the inner structure of the structured light generation unit 1, the structured light 110 is generated. When the light beams 111 of the structured light 110 is projected on a projection surface 14, a structured light pattern 15 corresponding to the structured light 110 is formed on the projection surface 14 in response to the plural light beams 111. That is, the structured light pattern 15 corresponding to the projection pattern 131 is formed on the projection surface 14. For example, the structured light pattern 15 is a grid pattern. As shown in FIG. 2, the structured light 110 outputted from the structured light generation unit 1 has a beam diffusion angle θ. Moreover, as the traveling distance of the light beams 111 of the structured light 110 increases, the coverage region of the structured light 2 is widened. That is, if the distance between the projection surface 14 and the structured light generation unit 1 is shorter, the structured light pattern 150a on the projection surface 14 is smaller (see FIG. 3A). Whereas, if the distance between the projection surface 14 and the structured light generation unit 1 is longer, the structured light pattern 15b on the projection surface 14 is larger (see FIG. 3B).
The selection of the size of the structured light pattern is determined according to the usage scenario of the user. Generally, the projected structured light pattern with high quality is required. That is, the efficiency of converting the incident light beams into the structured light should be as high as possible. In accordance with a key factor of increasing the efficiency, the beam distribution (or the light spot) of the plural light beams 111 from the light source 11 has to effectively cover the area of the projection pattern 131 of the diffractive optical element 13. In case that the beam distribution of the plural light beams 111 is slightly larger than or equal to the area of the projection pattern 131, the efficiency is high. Under this circumstance, the beam distribution of the plural light beams 111 can effectively match the diffractive optical element 13.
With increased development of science and technology, the electronic device should have slim appearance in order to meet the user's requirements. FIG. 4 is a schematic side view illustrating an inner portion of another conventional structured light generation unit. The structured light generation unit 2 as shown in FIG. 4 is a small-sized structured light generation unit. The structured light generation unit 2 comprises a light source 21, a collimating lens 22 and a diffractive optical element 23. Except that the collimating lens 22 is closer to the light source 21 and the thickness of the structured light generation unit 2 is smaller, the function and structure of the structured light generation unit 2 are substantially identical to those of the structured light generation unit 1. Although the structured light generation unit 2 is thinner, some drawbacks occur. For example, since the distance between the light source 21 and the collimating lens 22 is too short, the beam distribution of the plural light beams 211 incident on the diffractive optical element 23 is smaller. For allowing the beam distribution of the plural light beams 211 to effectively match the diffractive optical element 23, the area of the diffractive optical element 23 has to be reduced. Under this circumstance, the generated structured light pattern is decreased. Moreover, the diffractive optical element 23 with the smaller area indicates that the demand on the assembling tolerance of the optical apparatus is increased. In other words, the assembling complexity is increased.